Prosthetic knee joint

ABSTRACT

A prosthetic knee joint includes: (a) a femoral member comprising rigid material with a convex, wear-resistant femoral contact surface including a convex ridge; (b) a tibial assembly including: (i) at least one cup comprising rigid material with a body and a rim substantially thicker than the body defining a wear-resistant cup contact surface; (ii) a rigid base; and (iii) a spring support interconnecting the cup and the base, the spring support elastically deflectable permitting controlled pivoting of the at least one cup; (c) wherein the cup contact surface bears against the femoral contact surface, transferring axial and lateral loads between the cup and femoral member, while allowing pivoting between the cup and femoral member; and (d) wherein the at least one cup allows the rim to deform elastically, permitting the cup contact surface to conform in an irregular shape to the femoral contact surface, when the knee joint is loaded.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 12/983,191,filed Dec. 31, 2010, currently pending, which is a Continuation-in-Partof application Ser. No. 12/826,620, filed Jun. 29, 2010, now U.S. Pat.No. 7,914,580, which is a Continuation-in-Part of application Ser. No.12/714,288, filed Feb. 26, 2010, now U.S. Pat. No. 7,905,919, which is aContinuation-in-Part of application Ser. No. 11/936,601, filed Nov. 7,2007, currently pending, which claims the benefit of Provisional PatentApplication 60/864,667, filed on Nov. 7, 2006.

BACKGROUND OF THE INVENTION

This invention relates generally to medical implants, and moreparticularly to prosthetic joints having conformal geometries and wearresistant properties.

Medical implants, such as knee, hip, and spine orthopedic replacementjoints and other joints and implants have previously consisted primarilyof a hard metal motion element that engages a polymer contact pad. Thishas usually been a high density high wear resistant polymer, for exampleUltra-High Molecular Weight Polyethylene (UHMWPE), or other resilientmaterial. The problem with this type of configuration is the polymereventually begins to degrade due to the caustic nature of blood, thehigh impact load, and high number of load cycles. As the resilientmember degrades, pieces of polymer may be liberated into the joint area,often causing accelerated wear, implant damage, and tissue inflammationand harm.

It is desirable to employ a design using a hard member on a hard member(e.g. metals or ceramics), thus eliminating the polymer. Such a designis expected to have a longer service life. Extended implant life isimportant as it is now often required to revise or replace implants.Implant replacement is undesirable from a cost, inconvenience, patienthealth, and resource consumption standpoint.

Implants using two hard elements of conventional design will be,however, subject to rapid wear. First, a joint having one hard, rigidelement on another will not be perfectly shaped to a nominal geometry.Such imperfections will result in points of high stress, thus causinglocalized wear. Furthermore, two hard elements would lack the resilientnature of a natural joint. Natural cartilage has a definite resilientproperty, absorbing shock and distributing periodic elevated loads. Thisin turn extends the life of a natural joint and reduces stress onneighboring support bone and tissue. If two rigid members are used, thisability to absorb the shock of an active lifestyle could be diminished.The rigid members would transmit the excessive shock to the implant tobone interface. Some cyclical load in these areas stimulates bone growthand strength; however, excessive loads or shock stress or impulseloading the bone-to-implant interface will result in localized bone massloss, inflammation, and reduced support.

BRIEF SUMMARY OF THE INVENTION

These and other shortcomings of the prior art are addressed by thepresent invention, which provides a prosthetic joint havingwear-resistant contacting surfaces with conformal properties.

According to one aspect of the invention a prosthetic knee jointincludes: (a) a femoral member comprising a rigid material and having aconvex-curved, wear-resistant femoral contact surface, the femoralcontact surface including at least one protruding ridge having a convexcross-sectional shape; (b) a tibial assembly including: (i) at least onecup comprising a rigid material and having a body and a rim extendingaround an open periphery of the body, the rim being substantiallythicker than the body and defining a wear-resistant cup contact surface;(ii) a rigid base; and (iii) a spring support interconnecting at leastpart of the at least one cup and the base, the spring supportelastically deflectable so as to permit controlled pivoting motion ofthe at least one cup relative to the base; (c) wherein the cup contactsurface bears directly against the femoral contact surface, so as totransfer axial and lateral loads between the cup and the femoral member,while allowing pivoting motion between the tibial assembly and thefemoral member; and (d) wherein the at least one cup is shaped and sizedso as to allow the rim to deform elastically and permit the cup contactsurface to conform in an irregular shape to the femoral contact surface,when the knee joint is placed under a predetermined load.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figuresin which:

FIG. 1 is a cross-sectional view of a portion of a resilient contactmember constructed in accordance with the present invention;

FIG. 2 is an enlarged view of the contact member of FIG. 1 in contactwith a mating joint member;

FIG. 3 is a side view of a resilient contact member in contact with amating joint member;

FIG. 4 is a cross-sectional view of a cup for an implant according to analternate embodiment of the invention;

FIG. 5 is an enlarged view of a portion of the cup of FIG. 4;

FIG. 6 is a perspective view of a finite element model of a jointmember;

FIG. 7 is a cross-sectional view of an implant joint including aflexible seal;

FIG. 8 is an enlarged view of a portion of FIG. 7;

FIG. 9 is a side view of a prosthetic joint constructed in accordancewith an aspect of the present invention;

FIG. 10 is a cross-sectional view of the prosthetic joint of FIG. 9 inan unloaded condition;

FIG. 11 is a cross-sectional view of one of the members of theprosthetic joint of FIG. 9;

FIG. 12 is an enlarged view of a portion of FIG. 10;

FIG. 13 is a cross-sectional view of the prosthetic joint of FIG. 9 in aloaded condition;

FIG. 14 is an enlarged view of a portion of FIG. 13;

FIG. 15 is a cross-sectional view of an alternative joint member;

FIG. 16 is an enlarged view of a portion of FIG. 15;

FIG. 17 is a cross-sectional view of another alternative joint member;

FIG. 18 is a cross-sectional view of another alternative joint memberincluding a filler material;

FIG. 19 is a cross-sectional view of another alternative joint memberincluding a wiper seal;

FIG. 20 is a cross-sectional view of another alternative prostheticjoint;

FIG. 21 is a cross-sectional view of a prosthetic joint constructed inaccordance with another aspect of the present invention;

FIG. 22 is a cross-sectional view of a prosthetic joint constructed inaccordance with yet another aspect of the present invention; and

FIG. 23 is a perspective view of a joint member having a groovedsurface.

FIG. 24 is a exploded perspective view of two mating joint members;

FIG. 25 is a top plan view of one of the joint members shown in FIG. 24;

FIG. 26 is a cross-sectional view of one of the joint members shown inFIG. 24;

FIG. 27 is a contact stress plot of the joint member shown in FIG. 26;

FIG. 28 is a perspective view of a rigid joint member used forcomparison purposes;

FIG. 29 is a cross-sectional view of the joint member shown in FIG. 28;and

FIG. 30 is a contact stress plot of the joint member shown in FIG. 29;

FIG. 31 is a cross-sectional view of a prosthetic joint constructed inaccordance with another aspect of the present invention;

FIG. 32 is an enlarged view of a portion of the joint shown in FIG. 31;

FIG. 33 is a cross-sectional view of a cup member of the joint shown inFIG. 31;

FIG. 34 is a greatly enlarged cross-sectional view of a portion of thejoint shown in FIG. 31 in an initial condition;

FIG. 35 is a greatly enlarged cross-sectional view of a portion of thejoint shown in FIG. 31 after an initial wear-in period;

FIG. 36 is a graph showing contact pressure of the joint of FIG. 31compared to the number of operating cycles;

FIG. 37 is a cross-sectional view of a prosthetic joint constructed inaccordance with another aspect of the present invention; and

FIG. 38 is an enlarged view of a portion of the joint shown in FIG. 37.

FIG. 39 is a perspective view of a prosthetic knee joint;

FIG. 40 is a side elevational view of the joint of FIG. 39;

FIG. 41 is a rear elevational view of the joint of FIG. 39;

FIG. 42 is a perspective view of a tibial assembly of the joint of FIG.39;

FIG. 43 is a top view of the tibial assembly of FIG. 42;

FIG. 44 is a view taken along lines 44-44 of FIG. 43;

FIG. 45 is a perspective and sectional view of a first rimcross-sectional shape;

FIG. 46 is a perspective and sectional view of a second rimcross-sectional shape;

FIG. 47 is a perspective and sectional view of a third rimcross-sectional shape;

FIG. 48 is a perspective and sectional view of a fourth rimcross-sectional shape;

FIG. 49 is a perspective and sectional view of a first spring supportconfiguration;

FIG. 50 is a perspective and sectional view of a second spring supportconfiguration;

FIG. 51 is a perspective and sectional view of a third spring supportconfiguration;

FIG. 52 is a top view of an alternative tibial assembly;

FIG. 53 is a perspective view of the tibial assembly of FIG. 52;

FIG. 54 is a cross-sectional view taken along lines 54-54 of FIG. 51;

FIG. 55 is a top view of another alternative tibial assembly;

FIG. 56 is a perspective view of the tibial assembly of FIG. 55;

FIG. 57 is a cross-sectional view taken along lines 57-57 of FIG. 55;

FIG. 58 is a top plan view of a cup of the joint of FIG. 39;

FIG. 59 is a cross-sectional view of the cup of FIG. 58;

FIG. 60 is a cross-sectional view of an alternative tibial assembly;

FIG. 61 is a cross-sectional view along lines 61-61 of FIG. 40;

FIG. 62 is a cross-sectional view along lines 62-62 of FIG. 40; and

FIG. 63 is a cross-sectional view along lines 63-63 of FIG. 40.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a specialized implant contact interface(implant geometry). In this geometry, an implanted joint includes twotypically hard (i.e. metal or ceramic) members; however, at least one ofthe members is formed such that it has the characteristics of aresilient member, such as: the ability to absorb an impact load; theability to absorb high cycle loading (high endurance limit); the abilityto be self cleaning; and the ability to function as a hydrodynamicand/or hydrostatic bearing.

Generally, the contact resilient member is flexible enough to allowelastic deformation and avoid localized load increases, but not soflexible as to risk plastic deformation, cracking and failure. Inparticular, the resilient member is designed such that the stress levelstherein will be below the high-cycle fatigue endurance limit. As anexample, the resilient member might be only about 10% to about 20% asstiff as a comparable solid member. It is also possible to construct theresilient member geometry with a variable stiffness, i.e. having a loweffective spring rate for small deflections and a higher rate as thedeflections increase, to avoid failure under sudden heavy loads.

FIG. 1 illustrates an exemplary contact member 34 including a basicresilient interface geometry. The contact member 34 is representative ofa portion of a medical implant and is made of one or more metals orceramics (for example, partially stabilized Zirconia). It may be coatedas described below. The geometry includes a lead-in shape, Z1 and Z2, acontact shape, Z3 and Z4, a lead-out shape, Z5 and Z6, and a relievedshape, Z7. It may be desired to vary the cross-sectional thickness toachieve a desired mechanical stiffness to substrate resiliencecharacteristic. The presence of the relieved region Z7 introducesflexibility into the contact member 34, reduces the potential forconcentrated point contact with a mating curved member, and provides areservoir for a working fluid.

The Z7 region may be local to the contact member 34 or may be one ofseveral. In any case, it may contain a means of providing fluid pressureto the internal contact cavity to produce a hydrostatic interface. Apassive (powered by the regular motion of the patient) or active(powered by micro components and a dedicated subsystem) pumping meansand optional filtration may be employed to provide the desired fluidinteraction.

A hydrodynamic interface is desirable as, by definition, it means thecontact member 34 is not actually touching the mating joint member. Thelead-in and lead-out shapes Z1, Z2, Z5, Z6 are configured to generate ashear stress in the working fluid so as to create the fluid “wedge” of ahydrodynamic support.

FIG. 2 shows a closer view of the contact member 34. It may be desirableto make the contact radius (Z3 and Z4) larger or smaller, depending onthe application requirement and flexural requirement. For example, FIG.3 illustrates the contact member 34 in contact with a mating jointmember 38 having a substantially larger radius than the contact member34. The radius ratio between the two joint members is not particularlycritical, so long as one of the members exhibits the resilientproperties described herein.

The contact member 34 includes an osseointegration surface “S”, which isa surface designed to be infiltrated by bone growth to improve theconnection between the implant and the bone. Osseointegration surfacesmay be made from materials such as TRABECULAR METAL, textured metal, orsintered or extruded implant integration textures. TRABECULAR METAL isan open metal structure with a high porosity (e.g. about 80%) and isavailable from Zimmer, Inc., Warsaw, Ind. 46580 USA.

FIGS. 4 and 5 illustrate a cup 48 of metal or ceramic with twointegrally-formed contact rings 50. More contact rings may be added ifneeded. As shown in FIG. 5, the volume behind the contact rings 50 maybe relieved. This relieved area 52 may be shaped so as to produce adesired balance between resilience and stiffness. A varyingcross-section geometry defined by varying inner and outer spline shapesmay be desired. In other words, a constant thickness is not required. Amaterial such as a gel or non-Newtonian fluid (not shown) may bedisposed in the relieved area 52 to modify the stiffness and dampingcharacteristics of the contact rings 50 as needed for a particularapplication. The cup 48 could be used as a stand-alone portion of ajoint, or it could be positioned as a liner within a conventional liner.The contact ring 50 is shown under load in FIG. 6, which depicts contourlines of highest compressive stress at “C1”. This is the portion of thecontact ring 50 that would be expected to undergo bending first. Thebearing interface portion of the resilient contact member could beconstructed as a bridge cross-section supported on both sides as shownor as a cantilevered cross-section depending on the desired static anddynamic characteristics.

FIGS. 7 and 8 illustrate an implant 56 of rigid material which includesa wiper seal 58. The wiper seal 58 keeps particles out of the contactarea (seal void) 60 of the implant 58, and working fluid (natural orsynthetic) in. The seal geometry is intended to be representative and avariety of seal characteristics may be employed; such as a single lipseal, a double or multiple lip seal, a pad or wiper seal made from avariety of material options. Different seal mounting options may beused, for example a lobe in a shaped groove as shown in FIGS. 7 and 8, aretaining ring or clamp, or an adhesive. The wiper seal 58 may also beintegrated into the contact face of the interface zone.

It may be desirable to create a return passage 62 from the seal voidregion 60 back into the internal zone 64 in order to stabilize thepressure between the two and to allow for retention of the internal zonefluid if desired. This is especially relevant when the hydrostaticconfiguration is considered.

FIGS. 9-14 illustrate a prosthetic joint 100 comprising first and secondmembers 102 and 104. The illustrated prosthetic joint 100 isparticularly adapted for a spinal application, but it will be understoodthat the principles described herein may be applied to any type ofprosthetic joint. Both of the members 102 and 104 are bone-implantable,meaning they include osseointegration surfaces, labeled “S”, which aresurfaces designed to be infiltrated by bone growth to improve theconnection between the implant and the bone. Osseointegration surfacesmay be made from materials such as TRABECULAR METAL, textured metal, orsintered or extruded implant integration textures, as described above.As shown in FIG. 10, a central axis “A” passes through the centers ofthe first and second members 102 and 104 and is generally representativeof the direction in which external loads are applied to the joint 100 inuse. In the illustrated examples, the first and second joint members arebodies of revolution about this axis, but the principles of the presentinvention also extend to shapes that are not bodies of revolution.

The first member 102 includes a body 106 with a perimeter flange 116extending in a generally radially outward direction at one end.Optionally, a disk-like base 108 may be disposed at the end of the body106 opposite the flange 116, in which case a circumferential gap 111will be defined between the base 106 and the flange 116. The firstmember 102 is constructed from a rigid material. As used here, the term“rigid” refers to a material which has a high stiffness or modulus ofelasticity. Nonlimiting examples of rigid materials having appropriatestiffness for the purpose of the present invention include stainlesssteels, cobalt-chrome alloys, titanium, aluminum, and ceramics. By wayof further example, materials such as polymers would generally not beconsidered “rigid” for the purposes of the present invention. Generally,a rigid material should have a modulus of elasticity of about 0.5×10⁶psi or greater. Collectively, one end of the body 106 and the flange 116define a wear-resistant, concave first contact surface 118. As usedherein, the term “wear-resistant” refers to a material which isresistant to surface material loss when placed under load. Generally thewear rate should be no more than about 0.5 μm (0.000020 in.) to about1.0 μm (0.000040 in.) per million cycles when tested in accordance withASTM Guide F2423. As a point of reference, it is noted that any of thenatural joints in a human body can easily experience one millionoperating cycles per year. Nonlimiting examples of wear-resistantmaterials include solid metals and ceramics. Known coatings such astitanium nitride, chrome plating, carbon thin films, and/or diamond-likecarbon coatings may be used to impart wear resistance to the firstcontact surface 118. Optionally, the first contact surface 118 couldcomprise a separate face layer (not shown) of a wear-resistant materialsuch as ultra-high molecular weight (UHMW) polyurethane.

The first contact surface 118 includes a protruding peripheral rim 120(see FIG. 11), and a recessed central portion 122, which may also beconsidered a “pocket” or a “relief”. As used herein, the term “recessed”as applied to the central portion 122 means that the central portion 122lies outside of the nominal exterior surface of the second member 104when the joint 100 is assembled. In one configuration, shown in FIGS.9-14, and best seen in FIG. 11, the rim 120 is concave, with the radiusof curvature being quite high, such that the cross-sectional shape ofthe surface of the rim 120 approaches a straight line. FIGS. 15 and 16show another configuration of a joint member 102′ in which the rim 120′has a convex-curved cross-sectional shape. The cross-sectional shape ofthe rim may be flat or curved as necessary to suit a particularapplication.

The annular configuration of first contact surface 118 with theprotruding rim 120 results in a configuration which permits onlypivoting and rotational motion, and is statically and dynamicallydeterminate for the life of the joint 100. In contrast, prior artdesigns employing mating spherical shapes, even very accurate shapes,quickly reach a statically and dynamically indeterminate condition afteruse and wear. This condition accelerates wear, contributes to thefretting corrosion wear mechanism, and permits undesired lateraltranslation between the joint members.

The second member 104 is also made from a rigid material and has awear-resistant, convex second contact surface 124. The first and secondcontact surfaces 118 and 124 bear directly against each other so as totransfer axial and lateral loads from one member to the other whileallowing pivoting motion between the two members 102 and 104.

Nominally the first and second members 102 and 104 define a “ring” or“band” contact interface therebetween. In practice it is impossible toachieve surface profiles completely free of minor imperfections andvariations. If the first and second members 102 and 104 were bothcompletely rigid, this would cause high Hertzian contact stresses andrapid wear. Accordingly, an important feature of the illustrated joint100 is that the flange 116 (and thus the first contact surface 118) ofthe first member 102 is conformable to the second contact surface 124when the joint is placed under load.

FIGS. 10 and 12 show a cross-sectional view of the flange 116 in anunloaded condition or free shape. It can be seen that the distal end ofthe rim 120 contacts the second contact surface 124, while the inboardend of the rim 120 (i.e. near where the flange 116 joins the body 106)does not. FIGS. 13 and 14 show the flange 116 in a deflected position orloaded shape, where substantially the entire section width of the rim120 contacts the second contact surface 124, resulting in asubstantially increased contact surface area between the two members 102and 104, relative to the free shape. The rim 120′ of the joint member102′ (see FIG. 16) is similarly conformable; however, given the curvedcross-sectional shape, the total amount of surface contact area remainssubstantially constant in both loaded and unloaded conditions, with therim 120′ undergoing a “rolling” or “rocking” motion as the loadingchanges.

The conformable nature of the flange 116 is explained in more detailwith reference to FIGS. 24 through 30. As noted above, the first member102 has a flange 116 and a concave first contact surface 118. The secondmember 104 has a convex second contact surface 124. When assembled andin use the joint 100 is subject, among other loads, to axial loading inthe direction of the arrows labeled “F” in FIG. 24 (i.e. along axis “A”of FIG. 10). As previously stated, it is impossible in practice foreither of the contact surfaces 118 or 124 to be perfect surfaces (i.e. aperfect sphere or other curve or collection of curves). It is believedthat in most cases that a defect such as a protrusion from the nominalcontact surface of just 0.00127 mm (0.00005 in.), that is, 50 millionthsof a inch, or larger, would be sufficient to cause fretting corrosionand failure of a metal-on-metal joint constructed to prior artstandards. A defect may include a variance from a nominal surface shapeas well as a discontinuity in the contact surface. Defects may arisethrough a variety of sources such as manufacturing, installation, and/oroperating loads in the implanted joint.

FIG. 25 shows the second member 104 which in this particular examplevaries from a nominal shape in that it is elliptical rather thancircular in plan view. The elliptical shape is grossly exaggerated forillustrative purposes. For reference, the dimensions of the secondmember 104 along the major axis labeled “X” is about 0.0064 mm (0.00025in.) larger than its dimension along the minor axis labeled “Y”. Whenassembled and loaded, the flange 116 conforms to the imperfect secondcontact surface 124 and deflects in an irregular shape. In other words,in addition to any uniform deflection which may be present, thedeflected shape of the flange 116 includes one or more specificlocations or portions that are deflected towards or away from thenominal free shape to a greater or lesser degree than the remainder ofthe flange 116. Most typically the deflected shape would be expected tobe non-axisymmetric. For example, the deflection of the flange 116 atpoints located at approximately the three o'clock and nine o'clockpositions is substantially greater than the deflection of the remainderof the flange 116. As a result, the contact stress in that portion ofthe first contact surface 118 is relieved. FIG. 27 is a plan view plot(the orientation of which is shown by arrow in FIG. 26) whichgraphically illustrates the expected contact stresses in the firstcontact surface 118 as determined by analytical methods. The firstcontour line “C2” shows that a very low level of contract stress ispresent around the entire perimeter of the first contact surface 118.This is because the entire first contact surface 118 is in contact withthe second contact surface 124. Another contour line “C3” represents theareas of maximum contact stress corresponding to the protruding portionsof the elliptical second contact surface 124.

For comparative purposes, FIGS. 28 and 29 depict a member 902constructed according to prior art principles. The member 902 has acontact surface 918 with an identical profile and dimensions of thefirst contact surface 118 of the first member 102. However, consistentwith the prior art, the member 902 has a massive body 920 behind theentire contact surface 918, rendering the entire member 902substantially rigid. FIG. 30 graphically illustrates the expectedcontact stresses in the contact surface 918 as determined by analyticalmethods, when the member 902 is assembled and placed in contact with thesecond member 104, using the same applied load as depicted in FIG. 27.Because of the rigidity of the member 902, a “bridging” effect ispresent wherein contact between the contact surfaces (one of which iscircular in plan view, and the other of which is elliptical) effectivelyoccurs at only two points, located at approximately the three o'clockand nine o' clock positions. A first contour line “C4” shows twodiscrete areas where the lowest level of contract stress is present.These lines are not contiguous because there is no contact in theremaining area of the contact surfaces (for example at the six o'clockand twelve o'clock positions). Another contour line “C5” represents theareas of maximum contact stress. Analysis shows a peak contact stresshaving a magnitude of two to twenty times (or more) the peak contactstress of the inventive joint as shown in FIG. 27.

To achieve this controlled deflection, the flange 116 is thin enough topermit bending under working loads, but not so thin as to allow materialyield or fatigue cracking The deflection is opposed by the elasticity ofthe flange 116 in bending, as well as the hoop stresses in the flange116. To achieve long life, the first member 102 is sized so thatstresses in the flange 116 will be less than the endurance limit of thematerial, when a selected external load is applied. In this particularexample, the joint 100 is intended for use between two spinal vertebrae,and the design average axial working load is in the range of about 0 N(0 lbs) to about 1300 N (300 lbs.). These design working loads arederived from FDA-referenced ASTM and ISO standards for spinal discprostheses. In this example, the thickness of the flange 116, at a root126 where it joins the body 106 (see FIG. 12) is about 0.04 mm (0.015in.) to about 5.1 mm (0.200 in.), where the outside diameter of theflange 116 is about 6.4 mm (0.25 in.) to about 7.6 cm (3.0 in.).

The joint members may include multiple rims. For example, FIG. 17illustrates a joint member 202 where the first contact surface 218includes two protruding rims 220, with a circumferential groove orrelief area 228 therebetween. The presence of multiple rims increasesthe contact surface areas between the two joint members.

If present, the circumferential gap between the flange and the base ofthe joint member may be filled with resilient nonmetallic material toprovide damping and/or additional spring restoring force to the flange.FIG. 18 illustrates a joint member 302 with a filler 304 of this type.Examples of suitable resilient materials include polymers, natural orsynthetic rubbers, and the like.

As discussed above, the joint may incorporate a wiper seal. For example,FIG. 19 illustrates a joint member 402 with a resilient wiper seal 404protruding from the rim 420 of the first contact surface 418. The wiperseal 404 keeps particles out of the contact area (seal void), whilecontaining working fluid (natural or synthetic). The seal geometry isintended to be representative and a variety of seal characteristics maybe employed; such as a single lip seal, a double or multiple lip seal. Apad or wiper seal may be made from a variety of material options.Different seal mounting options may be used, for example a lobe inshaped groove as shown in FIG. 18, a retaining ring or clamp, adhesionsubstance. The seal may also be incorporated into the contact face ofthe interface zone.

The joint construction described above can be extended into a three-partconfiguration. For example, FIG. 20 illustrates a prosthetic joint 500having first, second, and third members 502, 504, and 506. The first andsecond members 502 and 504 are similar in construction to the firstmember 102 described above, and each includes a body 508, an optionaldisk-like base 510, and a flange 512. The flanges 512 definewear-resistant concave first and second contact surfaces 514 and 516,each of which includes a protruding peripheral rim, and a recessedcentral portion as described above. The third member 506 has adouble-convex shape defining opposed wear-resistant, convex third andfourth contact surfaces 524 and 526. The first and second 514 and 516bear against the third and fourth contact surfaces 524 and 526,respectively, so as to transfer axial (i.e. compression) and lateralloads between the first and second members 502 and 504 through the thirdmember 506, while allowing pivoting motion between the members 502, 504,and 506. The first and second contact surfaces 514 and 516 are conformalto the third and fourth contact surfaces 524 and 526 as described inmore detail above.

FIG. 21 illustrates an alternative prosthetic joint 600 comprising firstand second members 602 and 604 constructed from rigid materials. Both ofthe members 602 and 604 are bone-implantable, meaning they includeosseointegration surfaces, labeled “S”, as described in more detailabove.

The first member 602 is hollow and includes a disk-like base 606 and acup 608, interconnected by a peripheral wall 610. An interior cavity 612is defined between the base 606 and the cup 608. The cup 608 isconstructed from a rigid material and defines a wear-resistant, concavefirst contact surface 614. The first contact surface 614 includes aprotruding peripheral rim 616, and a recessed central portion 618, whichmay also be considered a “pocket” or a “relief”. The rim 616 may have aconical or curved cross-sectional shape.

The second member 604 is constructed from a rigid material and has awear-resistant, convex second contact surface 620. The first and secondcontact surfaces 614 and 616 bear directly against each other so as totransfer axial and laterals loads from one member to the other whileallowing pivoting motion between the two members 602 and 604.

As described above with reference to the prosthetic joint 100, the cup606 of the first member 602 is thin enough to permit bending underworking loads, but not so thin as to allow material yield or fatiguecracking. The first contact surface 614 is thus conformable to thesecond contact surface 620 when the prosthetic joint 600 is placed underexternal load.

An inverted configuration of hollow members is also possible. Forexample, FIG. 22 illustrates a prosthetic joint 700 comprising first andsecond members 702 and 704, both constructed of rigid materials. Thefirst member 702 is solid and includes a wear-resistant, concave firstcontact surface 708. The first contact surface 708 includes a protrudingperipheral rim 710, and a recessed central portion 712, which may alsobe considered a “pocket” or a “relief”.

The second member 704 is hollow and includes a dome 714 connected to aperipheral wall 716. An interior cavity 718 is defined behind the dome714. The dome 714 defines a wear-resistant, convex second contactsurface 720, which is shaped and sized enough to permit bending underworking loads, but not so as to allow material yield or fatiguecracking. The second contact surface 720 is thus conformable to thefirst contact surface 708 when the prosthetic joint 700 is placed underexternal load.

The first and second contact surfaces 708 and 720 bear directly againsteach other so as to transfer axial and lateral loads from one member tothe other while allowing pivoting motion between the two members 702 and704. Any of the contact surfaces described above may be provided withone or more grooves formed therein to facilitate flow of fluid ordebris. For example, FIG. 23 illustrates a joint member 800 including aconcave contact surface 802. The contact surface 802 includes a circulargroove 804, and plurality of generally radially-extending grooves 806which terminate at the center of the contact surface 802 and intersectthe circular groove 804.

FIGS. 31-33 illustrate an alternative prosthetic joint 1000 comprisingfirst and second members 1002 and 1004. The illustrated prosthetic joint1000 is particularly adapted for a ball-and-socket joint applicationsuch as is found in a human hip joint (i.e. the acetabulofemoral joint)or shoulder joint (i.e. the glenohumeral joint), but it will beunderstood that the principles described herein may be applied to anytype of prosthetic joint. Both of the members 1002 and 1004 arebone-implantable, meaning they include osseointegration surfaces,labeled “S”, which are surfaces designed to be infiltrated by bonegrowth to improve the connection between the implant and the bone.Osseointegration surfaces may be made from materials such as TRABECULARMETAL, textured metal, or sintered or extruded implant integrationtextures, as described above. As shown in FIG. 31, a nominal centralaxis “A” passes through the centers of the first and second members 1002and 1004 In the illustrated examples, the first and second joint members1002 and 1004 are bodies of revolution about this axis, but theprinciples of the present invention also extend to non-axisymmetricshapes.

The first member 1002 is constructed from a rigid material as describedabove. The first member 1002 is concave and may generally be thought ofas a “cup”, although it need not have any particular degree ofcurvature. Its interior defines a nominal cup surface 1006 shown by thedashed line in FIG. 33. The interior includes an annular first flange1008 which is located relatively near an apex 1010 of the first member1002 and which extends in a generally radial direction relative to theaxis A. The first flange 1008 is defined in part by an undercut groove1012 formed in the first member 1002. A ramped surface 1014 forms atransition from the groove 1012 to the nominal cup surface 1006. Thefirst flange 1008 includes a protruding first contact rim 1016. As usedherein, the term “protruding” as applied to the first contact rim 1016means that the first contact rim 1016 lies inside of the nominal cupsurface 1006 when the joint 1000 is assembled. The first contact rim1016 may have a curved or toroidal cross-sectional shape.

The interior also includes an annular second flange 1018 which islocated at or near an outer peripheral edge 1020 of the first member1002 and which extends in a generally axial direction relative to theaxis A. The second flange 1018 is defined in part by an undercut groove1022 formed in the first member 1002. The second flange 1018 includes aprotruding second contact rim 1024. As used herein, the term“protruding” as applied to the second contact rim 1024 means that thesecond contact rim 1024 lies inside of the nominal cup surface 1006 whenthe joint 1000 is assembled. The second contact rim 1024 may have acurved or toroidal cross-sectional shape. Depending on the particularapplication, joint 1000 may include more than two flanges defining morethan two contact rims.

In the illustrated example, the first member 1002 includes a face layer1026 of a known coating such as titanium nitride, chrome plating, carbonthin films, and/or diamond-like carbon coatings, and/or a anotherwear-resistant material such as ultra-high molecular weight (UHMW)polyurethane. This face layer 1026 is used to impart wear resistance, asdescribed above. The face layer 1026 may be extraordinarily thin. Inthis particular example, its as-applied thickness is about 0.0041 mm(0.00016 in.), or 160 millionths of a inch thick. The face layer 1026 isapplied at a substantially uniform thickness over the surface profilewhich is defined by machined or formed features of the substrate.Alternatively, and especially if a much thicker face layer were used,the face layer could be profiled so as to define both the nominal cupsurface 1006 and the first and second contact rims 1016 and 1024.

The second member 1004 is also made from a rigid material and has awear-resistant, convex contact surface 1028. In the specific exampleillustrated, the second member 1004 includes a face layer 1030 of aknown coating such as titanium nitride, chrome plating, carbon thinfilms, and/or diamond-like carbon coatings, and/or a anotherwear-resistant material such as ultra-high molecular weight (UHMW)polyurethane. This face layer 1030 is used to impart wear resistance,and may be quite thin, as described above. The first and second contactrims 1016 and 1024 bear directly against the contact surface 1028 so asto transfer axial and lateral loads from one member to the other whileallowing pivoting motion between the two members 1002 and 1004.

The annular configuration of contact rims 1016 and 1024 results in ajoint configuration which permits only pivoting and rotational motion,and is statically and dynamically determinate for the life of the joint1000. In particular, the presence of the relatively widely-spacedcontact rims 1016 and 1024, and the peripheral positioning of the secondcontact rim 1024 is highly effective in resisting any translation of thefirst and second members 1002 and 1004 lateral to the axis A.

Nominally the first and second contact rims 1016 and 1024 define twoseparate “ring” or “band” contact interfaces with the contact surface1028 of the second member 1004. In practice it is impossible to achievesurface profiles completely free of minor imperfections and variations.If the first and second members 1002 and 1004 were both completelyrigid, this would cause high Hertzian contact stresses (i.e. non-uniformcontact) and rapid wear. Accordingly, an important feature of theillustrated joint 1000 is that the flanges 1008 and 1018 (and thus thecontact rims 1016 and 1024) of the first member 1002 are conformable tothe contact surface 1028 when the joint 1000 is placed under load. Theflanges 1008 and 1018 can conform to the imperfect contact surface 1028and deflect in an irregular shape. In other words, in addition to anyuniform deflection which may be present, the deflected shape of theflanges 1008 and 1018 can include one or more specific locations orportions that are deflected towards or away from the nominal free shapeto a greater or lesser degree than the remainder of the flanges 1008 and1018. To achieve this controlled deflection, the flanges 1008 and 1018are thin enough to permit bending under working loads, but not so thinas to allow material yield or fatigue cracking, or to exceed theendurance limit of the material. The deflection is opposed by theelasticity of the flanges 1008 and 1018 in bending, as well as the hoopstresses in the flanges 1008 and 1018.

The contact rims 1016 and 1024 are designed in conjunction with thecontact surface 1028 to create a wear characteristic that is constantlydiminishing (similar to an asymptotic characteristic). With reference toFIG. 32, the as-manufactured or initial curvatures (e.g. radii) of thefirst and second contact rims 1016 and 1024, denoted “R” are differentfrom the curvature (e.g. radius) of the contact surface 1028, denoted“r”. It is noted that the direction of curvature (i.e. the convexity orsecond derivative shape) of the first and second contact rims 1016 and1024 may be the same as, or opposite to, that of the contact surface1028 upon initial manufacture. In this example they are opposite. Whenassembled and placed under load, the annular interface between each ofthe contact rims 1016 and 1024 and the contact surface 1028 will have acharacteristic width denoted “W”, (effectively creating a contact band).The initial dimensions R and r are selected such that, even using highlywear-resistant surfaces or coatings, some wear takes place during aninitial wear-in period of movement cycles. As a result, the contact bandwidth W increases during the initial wear-in period. This increasescontact area and therefore decreases contact stress for a given load.After the initial wear-in period (which preferably occurs before thejoint is implanted), the contact band reaches a post wear-in width atwhich the contact stress is below a selected limit, below which the rateof wear in the contacting surfaces approaches a very low number or zero,consistent with a long life of the joint 1000. FIG. 36 illustrates thiswear characteristic, with the limit “L” depicted as a horizontal line.

FIGS. 34 and 35 are schematic views showing the initial wear-in of thesurface of the contact rim 1016 at a microscopic (or nearly microscopic)level. It will be understood that these figures are greatly exaggeratedfor the purposes of illustration. On initial manufacture, as shown inFIG. 34, the curvatures R and r of the contact rim 1016 and the contactsurface 1028 have opposite directions. When assembled, the contact bandwidth W is some nominal value, for example about 0.03 mm (0.001 in.),and the total thickness “T” of the face layer 1026 is at its as-appliedvalue of about 0.0041 mm (0.00016 in.) for example. The action of thewear-in period described causes the face layer 1026 to wear to a shapecomplementary to the contact surface 1028. After this wear-in period thecurvature of the portion of the contact rim 1016 within the contactband, denoted “R′”, and the curvature r of the contact surface 1028 arein the same direction, and the values of the two curvatures aresubstantially the same. For example, the thickness T at the location ofthe contact band may decrease by about 0.0004 mm (0.000014 in.), with acorresponding increase in the width of the contact band W to about 0.2mm (0.008 in.). Analysis shows that this increase in contact band widthand surface area can reduce mean contact pressure by over 80%.

The configuration of the flanges 1008 and 1018 are important indeveloping the constantly diminishing wear characteristics describedabove. In particular, the flanges 1008 and 1018 are sized and shaped sothat deflections of the contact rims 1016 and 1024 under varying loadare always essentially normal to their respective tangent points on theopposing contact surface 1028, as the joint 1000 is loaded and unloaded.This ensures that the position of each of the contact bands remainsconstant and that the contact bands remain substantially uniform aroundthe entire periphery of the joint 1000.

An inverted configuration of the joint described above may be used. Forexample, FIGS. 37 and 38 illustrate a prosthetic joint 1100 having firstand second members 1102 and 1104 which are substantially similar ingeneral construction to the members of the joint 1000 described above interms of materials, coatings, and so for forth. However, in this joint1100, the concave member 1102 has a contact surface without protrudingrings. The convex member 1104 has first and second flanges 1108 and 1118which define first and second contact rims 1116 and 1124 which functionin the same manner that the flanges and contact rims described above.

FIGS. 39-44 illustrate a prosthetic knee joint 2000. While theillustrated prosthetic joint 2000 is particularly adapted to replace thenatural joint application such found in a human knee, it will beunderstood that the principles described herein may be applied to anytype of prosthetic joint which requires similar movements. The maincomponents of the joint 2000 are a femoral member 2002 and a tibialassembly 2004.

The femoral member 2002 is constructed from a rigid material asdescribed above. The femoral member 2002 is generally “U”-shaped in sideelevation. One leg of the “U” shape is divided by a slot 2006 into twoseparate, parallel fingers 2008. The femoral member 2002 includes aninner surface 2010 which is shaped for convenient and secure attachmentto the lower end of a patient's femur (not shown). In this particularexample the inner surface 2010 comprises a series of planar surfaces orfacets. The inner surface 2010 may be configured as an osseointegrationsurface, i.e., a surface designed to be infiltrated by bone growth toimprove the connection between the implant and the bone.Osseointegration surfaces may be made from materials such as TRABECULARMETAL, textured metal, or sintered or extruded implant integrationtextures, as described above. The femoral member 2002 may be mounted toa patient's femur using a press fit in a known manner, or partially orfully cemented to the femur in a known manner. The outer surface of thefemoral member 2002 is a femoral contact surface 2012. Its surfaceprofile includes two protruding ridges 2014 (see FIG. 41) each having aconvex cross-sectional shape. Each ridge 2014 runs along one of thefingers 2008 and extends around the “U” shape. These ridges 2014correspond to the lateral and medial condyles of a human femur and maybe interchangeably referred to herein as “condyles”. The curvature ofthe “U” shape in side elevation view (and thus the curvature of theridges 2014 along their arc length), indicated schematically at “C” inFIG. 40, may be a constant or variable radius.

The femoral contact surface 2012 is wear-resistant, as that term isdefined above. In the specific example illustrated, the femoral contactsurface 2012 includes a face layer (not depicted separately) of a knowncoating such as titanium nitride, chrome plating, carbon thin films,and/or diamond-like carbon coatings, and/or a another wear-resistantmaterial such as ultra-high molecular weight (UHMW) polyurethane. Thisface layer is used to impart wear resistance, and may be quite thin, asdescribed above.

The tibial assembly 2004 is shown in more detail in FIGS. 42-44. Itincludes cups 2016, spring supports 2018, and a base 2020. The base 2020is made from a rigid material and includes a generally flat plate 2022with open-ended cylindrical receptacles 2024. Collectively the plate2022 and the exterior of the receptacles 2024 define a mounting surface2026. The mounting surface 2026 may be configured as an osseointegrationsurface, i.e. a surface designed to be infiltrated by bone growth toimprove the connection between the implant and the bone.Osseointegration surfaces may be made from materials such as TRABECULARMETAL, textured metal, or sintered or extruded implant integrationtextures, as described above. The mounting surface 2026 and thereceptacles 2024 are intended to be rigidly mounted to the upper end ofa patient's tibia (not shown) and are sized and shaped for that purpose.The base 2020 may be mounted to a tibia using a press fit in a knownmanner, or partially or fully cemented to the tibia in a known manner.

Each cup 2016 is made from a rigid material (as described above) andincludes a central disk 2028, a concave body 2030 extending upwards fromthe disk 2028, and a rim 2032 extending around the open periphery of thebody 2030. The rim 2032 protrudes laterally away from (i.e. inboard andoutboard) from the body 2028 and thus may be described as beingsubstantially “thicker” or “wider” than the body 2030. The rim 2032 hasa saddle-like or “ribbon” shape which is generally oval in plan view. Asused herein, the term “oval” refers to a shape which has an overalllength along one axis of symmetry (e.g. the x-axis seen in FIG. 42) thatis substantially longer than an overall length along a second,perpendicular axis of symmetry (e.g. the y-axis seen in FIG. 42). Inside elevation view, the saddle-like shape is “lowest” in the middle(relative to the central disk 2028) and curved upward at the ends. It isnoted that the saddle-like shape may be oriented in any directionrelative to the z-axis. That is, the “lowest” portion of the saddle-likeshape could face in medial, lateral, anterior, or posterior directions.In the illustrated example there is one cup 2016 for each ridge 2014,however the tibial assembly 2004 could be modified to include more thanone cup 2016 for each ridge 2014.

In the example shown in FIGS. 39-44, the rim 2032 is formed by a rolled,generally circular cross-sectional shape. The rim 2032 may take on avariety of shapes to adjust or “tune” its specific behavior in responseto loading. Nonlimiting examples of potential cross-sectional shapes forthe rim 2032 are shown in FIGS. 45 through 48 as follows: open circular(FIG. 45); solid circular (FIG. 46); solid generally circular with aconvex bearing portion (FIG. 47); and solid generally circular with aconcave bearing portion (FIG. 48).

The rim 2032 is wear-resistant, as that term is defined above. In thespecific example illustrated, the rim 2032 includes a face layer (notseparately indicated) of a known coating such as titanium nitride,chrome plating, carbon thin films, and/or diamond-like carbon coatings,and/or a another wear-resistant material such as ultra-high molecularweight (UHMW) polyurethane. This face layer is used to impart wearresistance, and may be extraordinarily thin. In this particular example,its as-applied thickness is about 0.0041 mm (0.00016 in.), or 160millionths of a inch thick. The face layer is applied at a substantiallyuniform thickness over the surface over the rim 2032.

Each cup 2016 is connected to the base 2020 by one of the springsupports 2018. The spring supports 2018 are received in the receptacles2024 and may be connected to the cups 2016 and the base 2020 by meanssuch as welding or brazing, mechanical fasteners, or adhesives. Thespring supports 2018 are configured to be substantially resistant totorsion (that is, rotation of the cup 2016 about the z-axis as shown inFIG. 42), but to permit controlled pivoting of the cup 2016 about the x-and y-axes so as to conform to the femoral component 2002, as describedin more detail below. The spring supports 2018 are also substantiallyresistant to compression in the z-direction, having a very higheffective spring rate. As used to describe the characteristics of thespring supports 2018, the term “substantially resistant” implies aspring rate or modulus sufficiently high so as to permit no significantdeflection under normally-expected applied loads.

Various types of structures may be used for the spring supports 2018. Inthe example shown in FIGS. 39-44, and also in FIG. 49, the springsupport 2018 is a bellows-like structure formed from sheet metal, withan annular wall having a plurality of convolutions. Other potentialexamples are a monolithic cylindrical structure with a plurality ofslots selectively cut or otherwise formed therein, shown in FIG. 50, ora structure formed from a stack of conical spring washers (commonlyknown as “Belleville washers”), shown in FIG. 51. The materialproperties and overall dimensions of the spring support 2018 as well asdetails such as the number of slots, convolutions, or washers, may bevaried to provide the spring characteristics needed for a particularapplication.

When assembled as shown in FIGS. 39-41, each of the rims 2032 bearsagainst one of the ridges 2014. The spring supports 2018 elasticallydeflect and permit the cups 2016 to tilt (i.e. pivot about the x- andy-axes) so that the rims 2032 can engage the ridges 2014 at aappropriate angle. The joint 2000 permits flexion and extensionmovements (generally indicated by the heavy arrow in FIG. 40) by slidingof the ridges 2014 relative to the rims 2032. The joint 2000 alsoaccommodates slight relative rotation of the femoral component 2002 andthe tibial assembly 2004 about the z-axis (generally indicated by theheavy arrow in FIG. 39). This motion is accommodated throughdifferential pivoting of the spring supports 2018 (i.e. positivepivoting of one spring support 2018 about the y-axis and negativepivoting of the other spring support 2018 about the y-axis).

FIGS. 52-54 depict an alternative tibial assembly 2104 for use with thefemoral component 2002. It is similar to the tibial assembly 2004described above and includes cups 2116, spring supports 2118, and a base2120. The base 2120 is made from a rigid material and includes agenerally flat plate 2122 with open-ended cylindrical receptacles 2124.The receptacles 2124 are significantly shallower than the receptacles2024 described above, and house spring supports 2118 which arecorrespondingly shorter than the spring supports 2018 shown in FIGS.42-44. A pin 2125 extends downward from the bottom of each receptacle2124. It is noted that, depending on the particular application, thereceptacles 2124 may be completely eliminated, with the spring support2118 extending between a top surface of the plate 2122 and the cup 2116(i.e. partially or fully taking the place of the central disk 2028described above). Collectively the plate 2122 and the exterior of thereceptacles 2124 and pins 2125 define a mounting surface 2126. Themounting surface 2126 may be configured as an osseointegration surface,i.e. a surface designed to be infiltrated by bone growth to improve theconnection between the implant and the bone. Osseointegration surfacesmay be made from materials such as TRABECULAR METAL, textured metal, orsintered or extruded implant integration textures, as described above.The base 2220 may be mounted to a tibia using a press fit in a knownmanner, or partially or fully cemented to the tibia in a known manner.

FIG. 60 depicts an alternative tibial assembly 2304 for use with thefemoral component 2002. It includes cups 2316 mounted to a base 2320.The base 2320 is made from a rigid material and includes a generallyflat plate 2322. Each spring support 2318 is integral to the plate 2322and comprises a bellows-like structure formed from sheet metal, with anannular wall having a plurality of convolutions. A central stem 2325extends upward from the hollow center of each spring support 2318 tohold the corresponding cup 2316. Collectively the plate 2322 and theexterior of the spring elements 2318 define a mounting surface 2326 asdescribed above.

FIGS. 55-57 depict yet another an alternative tibial assembly 2204 foruse with the femoral component 2002. It includes cups 2216 mounted to abase 2220. The base 2220 is made from a rigid material and includes agenerally flat plate 2222 with one or more pins 2225 extending downwardfrom a lower surface thereof. Collectively the plate 2222 and theexterior of pins 2225 define a mounting surface 2226. The mountingsurface 2226 may be configured as an osseointegration surface, i.e. asurface designed to be infiltrated by bone growth to improve theconnection between the implant and the bone. Osseointegration surfacesmay be made from materials such as TRABECULAR METAL, textured metal, orsintered or extruded implant integration textures, as described above.The base 2220 may be mounted to a tibia using a press fit in a knownmanner, or partially or fully cemented to the tibia in a known manner.

In this tibial assembly 2204, the spring support forms an integral partof each cup 2216. In particular, the walls of the cup 2216 are shapedwith one or more discrete elastically deflectable curved portions 2217(such as “S” or “C” curves) so as to permit the controlled pivotingmotion described above with respect to the spring supports 2018.

Nominally, when the joint 2000 is assembled and placed under load, eachrim 2032 tends to define two separate “contact bands” with the opposedridge 2014. Exemplary contact bands “B” are shown schematically in FIG.58. Depending on the specific dimensions and curvatures of the femoralmember 2002 and the cups 2016, the bands B may be a different size thandepicted, or merge into a single complete annular contact band. As thejoint 2000 is loaded, downward deflection of the rim 2032 is opposed bythe elasticity of the rims 2032 in bending, as well as the hoop stressesin the rims 2032.

The deflection properties of the rims 2032 are further controlled bytheir three-dimensional “saddle” shape. Each rim 2032 can be conceivedof as a “ribbon” in space which has its own bending and torsionalrigidity, separate from the support provided by the cup 2016. As aresult, vertical deflection (i.e. along the z-axis) of the rim 2032,shown schematically by the heavy arrows in FIG. 59, results in a linkedhorizontal deflection (i.e. along the y-axis), shown schematically bythe heavy arrows in FIG. 58. In other words, loading or unloading of therim 2032 tends to cause expansion or contraction of the rim 2032 in planview. This property tends to make the contact bands B remain tangent tothe femoral contact surface 2012 at all times. This function isimportant in making the contract bands B remain at a stable position onthe surface of the rim 2032, resulting in very low wear and long life.

Further control of the deflection properties of the rims 2032 may beexercised through the interaction of the curvatures of the ridges 2014and the rims 2032. In particular, the curvature or width of the ridges2014 parallel to the y-axis as shown in FIG. 39 may be varied. Forexample, FIGS. 61-63 show cross-sectional cuts taken through threedifferent cutting planes depicted in FIG. 40. Explained another way, theridges 2014 have variable lateral cross-sectional shapes at variouslocations along the arc length of the overall “U” curvature. Interactionof larger or smaller curvatures or widths of the ridges 2014 as thejoint 2000 moves causes corresponding larger or smaller deflections inthe rims 2032, in turn exerting control over the motion of the rims 2032and further enhancing the ability to make the contact bands B remaintangent to the femoral contact surface 2012 at all times.

Also, in practice it is impossible to achieve surface profilescompletely free of minor imperfections and variations. If the ridge 2014and rim 2032 were both completely rigid, this would cause high Hertziancontact stresses (i.e. non-uniform contact) and rapid wear. Accordingly,an important feature of the illustrated joint 2000 is that the rims 2032are conformable to the ridges 2014 when the joint 2000 is placed underload. The rims 2032 can conform to the imperfect femoral contact surface2012 and deflect in an irregular shape. In other words, in addition toany uniform deflection which may be present, the deflected shape of therims 2032 can include one or more specific locations or portions thatare deflected towards or away from the nominal free shape to a greateror lesser degree than the remainder of the rims 2032. To achieve thiscontrolled deflection, the cups 2016 and rims 2032 are thin enough topermit bending under working loads, but not so thin as to allow materialyield or fatigue cracking, or to exceed the endurance limit of thematerial. The deflection is opposed by the elasticity of the rims 2032in bending, as well as the hoop stresses in the rims 2032.

As noted above, known coatings such as titanium nitride, chrome plating,carbon thin films, and/or diamond-like carbon coatings may be used toimpart wear resistance or augment the wear resistance of any of thecontact surfaces and/or contact rims described above. To the same end,it may be desirable to surface treat either or both interfaces of any ofthe above-described implants or joints with a laser, shot peen,burnishing, or water shock process, to impart residual compressivestresses and reduce wear. The benefit could be as much from surfaceannealing and microstructure and microfracture elimination as smoothingitself.

The foregoing has described prosthetic joints with wear-resistantproperties and conformal geometries. While specific embodiments of thepresent invention have been described, it will be apparent to thoseskilled in the art that various modifications thereto can be madewithout departing from the spirit and scope of the invention.Accordingly, the foregoing description of the preferred embodiment ofthe invention and the best mode for practicing the invention areprovided for the purpose of illustration only and not for the purpose oflimitation.

1. A prosthetic knee joint, comprising: (a) a femoral member comprisinga rigid material and having a convex-curved, wear-resistant femoralcontact surface, the femoral contact surface including two spaced-apartprotruding ridges having a convex cross-sectional shape; and (b) atibial assembly including: (i) two individual spaced-apart cups, eachcomprising a rigid material and having a body and a rim extending aroundan open periphery of the body, the rim being substantially thicker thanthe body and defining a wear-resistant cup contact surface, wherein therim has an elevational profile shape which is elevated at distal endsthereof, and lower in a middle thereof, relative to the body; (ii) arigid base; and (iii) two separate spaced-apart spring supports, eachspring support interconnecting at least part of one of the cups to thebase, wherein each spring support is elastically deflectable so as topermit controlled pivoting motion of the respective cup relative to thebase; (c) wherein each of the cup contact surfaces bears directlyagainst the femoral contact surface, so as to transfer axial and lateralloads between the tibial assembly and the femoral member, while allowingpivoting motion between the tibial assembly and the femoral member. 2.The prosthetic knee joint of claim 1, wherein each cup has a plan shapewhich is generally oval, having an overall length along one axis ofsymmetry which is substantially longer than an overall length along asecond axis of symmetry perpendicular to the first axis of symmetry. 3.The prosthetic knee joint of claim 1 wherein each cup is shaped andsized so as to allow its rim to deform elastically and permit the cupcontact surface to conform in an irregular shape to the femoral contactsurface, when the knee joint is placed under a predetermined load. 4.The prosthetic knee joint of claim 1 wherein each spring supportcomprises a monolithic cylindrical structure with a plurality of slotsformed therein.
 5. The prosthetic knee joint of claim 1 wherein eachspring support comprises at least one conical spring washer.
 6. Theprosthetic knee joint of claim 1 wherein each spring support comprises agenerally cylindrical structure having an annular wall with a pluralityof convolutions formed therein.
 7. The prosthetic knee joint of claim 1wherein each spring support is received within a receptacle formed inthe base.
 8. The prosthetic knee joint of claim 1 wherein each springsupport comprises at least one discrete elastically deflectable curvedportion included in the body of each cup.
 9. The prosthetic knee jointof claim 1, wherein all of the contact surfaces are ceramic, metallic,or a combination thereof.
 10. The prosthetic knee joint of claim 1,where each cup is sized so as to permit elastic deflection of the cupwhile limiting stresses in the cup to less than the endurance limit ofthe material, when a predetermined external axial load is applied to thejoint.
 11. The prosthetic joint of claim 1 wherein at least one of thecontact surfaces includes a wear-resistant thin film or coating.
 12. Theprosthetic joint of claim 1 wherein each spring support comprises agenerally cylindrical structure having an annular wall with a pluralityof convolutions formed therein, and a central stem extending upwardsthrough a center of the annular wall to connect to the respective cup.13. The prosthetic joint of claim 1 wherein each ridge has variablelateral cross-sectional curvatures at spaced-apart locations along anarc length of each ridge.